[SPARK-47993][DOCS][FOLLOWUP] Update RDD programing guide to Python 3.9+

[dongjoon-hyun]

What changes were proposed in this pull request?

This is a follow-up of

• [SPARK-47993][PYTHON] Drop Python 3.8 #46228

Why are the changes needed?

To update the RDD programing guide consistently.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Manual review.

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

This seems to be the last place which we missed.

$ git grep 'Python 3.8'
licenses-binary/LICENSE-loose-version.txt:Starting with Python 3.8.6, examples, recipes, and other code in
licenses/LICENSE-loose-version.txt:Starting with Python 3.8.6, examples, recipes, and other code in
python/docs/source/migration_guide/pyspark_upgrade.rst:* In Spark 4.0, Python 3.8 support was dropped in PySpark.
python/pyspark/cloudpickle/cloudpickle.py:            # backport of the Python 3.8 state_setter pickle operations
python/pyspark/serializers.py:    # For Python 3.8+, we use CPickle-based cloudpickle.

[dongjoon-hyun]

cc @HyukjinKwon

[HyukjinKwon]

Merged to master.

[dongjoon-hyun]

Thank you, @HyukjinKwon !